Friday, February 13, 2015

Friday afternoon update on President's Day winter weather potential

First things first, enjoy tomorrow! It should turn out to be a very pleasant late winter day with highs in the 50s - and the last time we see temps anywhere close to that for nearly a week.

By late Saturday afternoon, the next in a series of Arctic cold fronts arrives. Wind turns northwesterly and becomes strong during the evening and overnight hours. The front will drop temps overnight to near 20. Temperatures Sunday will struggle to reach freezing.  It'll be a raw day with wind chills in the teens. First ingredient for our potential winter weather - cold air. Check.

Sunday morning will feel downright raw with wind chill values in the single digits.

Starting Sunday night is when things get interesting. Yesterday we discussed the possibility of a dual threat Monday and Tuesday. Since then, models have come into fair agreement that the first wave will be of much more concern, and that one looks to impact the area Sunday night into early Monday. There is still some discrepancy on the exact timing, but much of it will be overnight Sunday night and it could linger into the mid-morning hours Monday. The second wave, Tuesday, now looks to be much less of a problem as high pressure will briefly build in behind the first wave, forcing all precip well south of us for the second half of Monday and Tuesday.

Total (liquid/melted) precip amounts through Monday morning from the latest NAM model show 0.3-0.4" for the metro. Some of this would fall as sleet and some as snow. The longer it sleets, the faster snow amounts decline. These amounts appear reasonable to us at this time.

What to expect

So what do we expect Sunday night/early Monday? With cold (below freezing) dry air in place and the possibility of a bit of warm air aloft, precip could start as sleet, or a sleet/snow mix, Sunday late evening. Currently, we believe onset will probably be in the 9pm-midnight timeframe. Overnight, it appears precip becomes steadier and becomes all snow. With temps in the 20s, the snow would be a little fluffier than usual for this part of the country, which serves to increase snow totals a bit given the same amount of liquid water. Precip end time is up for debate, but should be between dawn and mid-morning Monday. How much? After careful analysis and watching trends in the data, our first, very early, educated guess is up to 1/4" of sleet and 1-2" of snow in the immediate metro. Higher amounts are possible north and lower amounts south. However, we have recent model data that supports more sleet, more snow, and less snow.  This may not seem like a lot, but for the Memphis area, it's enough to generate impacts, especially to travel.

NWS graphic showing the probability of 1" of snow or greater through Monday 6am. Since precip could be ongoing, the numbers are probably a bit low. Areas north of I-40 have a greater chance of seeing more snow.

If you're a model-watcher, our forecast is down some from what models were depicting yesterday. This outlook is likely not exactly right and WILL change between now and Sunday night - maybe higher, maybe lower. Please continue to monitor our updates the next 2 days as things becomes more certain - don't take this initial forecast as gospel. (We're talking about southern winter weather here!)

Potential impacts

If the above forecast verifies, Monday morning rush hour WOULD be impacted. Fortunately it's a federal holiday and many schools are out so traffic will be lighter. Since we're not currently expecting freezing rain, ice accumulation on objects (power lines, trees, etc.) is not expected to be a big deal. However bridges will be slick and there will be fewer cars on the road to warm them up and melt whatever is there.

Given that, the situation that developed last year in Birmingham and Atlanta is possible in this setup. In that case, a light amount of snow fell in very cold temperatures (just a bit colder than what we're expecting). The combination of tires melting the snow and very cold air re-freezing the snowmelt resulted in icy streets. Be aware of the possibility and plan to allow extra time if you have a Monday commute. A huge rush on the grocery stores is not necessary at this point (see, our forecast isn't sponsored by Kroger!).

Note also that behind this system, very cold air will be in place with some data indicating we may barely get above freezing from Saturday evening through Friday of next week. The coldest period will be Wednesday morning when lows could drop into the single digits. Anything that falls could stick around a while, though roads with traffic would improve during the day Monday as precip ends.

That leads me to these points:

  1. There's typically a lot of sharing of winter weather graphics on social media, especially those with "extreme" snowfall predictions. BEWARE THE SHARE! If it doesn't come from a source that you trust and that can EXPLAIN it to you (most people can't explain the nuances of a snow accumulation map), disregard and don't share!
  2. In addition, computer models make most of the maps people share. They're fallible, and they change regularly. What was valid 6 hours ago is not any longer. Trust the human forecaster with experience, not the computer model graphic, unless then human says "I think this is right" (as in the case above where we shared the NAM model graphic). Anyone (literally) can find a model "solution" that is an outlier and post it as a forecast. See the excellent graphic below, put together by NWS-Fort Worth, with more tips on using social media before and during winter weather.
  3. Despite many people's sworn belief in their powers, the bluffs, the Pyramid, the "snow dome,"and the Indian burial grounds have NOTHING to do with the weather. They don't deflect snow and they don't protect us from tornadoes. I KNOW it's been a long time since we had a "good snow." It's luck of the draw and climatology of the area. It's just not very common in this area. Yes, we're due. Maybe this one is it. I don't know. The graphics and memes are cute and it's nice to have fun with them, but know that there is no truth in any of those myths

We'll continuously monitor the latest data and keep you apprised as necessary to any changes. We're also planning a Google+/YouTube video chat this weekend, tentatively set for Sunday at 6:30pm to discuss the latest expectations and impacts. Monitor our social media accounts for more details on that informative discussion.

Erik Proseus
MWN Meteorologist

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